Side looking sar system

ABSTRACT

The invention relates to a side-looking SAR system, comprising  
     a transmit aperture,  
     a receive aperture of different size, separated from said transmit aperture and divided into a number of receive sub-apertures arranged along elevation and azimuth direction,  
     means for coherently processing the signals of each receive sub-aperture comprising  
     means for phase shifting the signal from each receive sub-aperture by a time and/or frequency variant phase value,  
     means for the summation of the resulting signals from receive sub-apertures arrayed along the elevation direction,  
     whereby the time and/or frequency variant phase value is generated in such a way that the radar echo signal is maximized in the summed signal as the radar transmit signal runs over the earth&#39;s surface.

[0001] The invention relates to a side-looking SAR (synthetic apertureradar) system.

[0002] 1. Limitations of a Conventional SAR System

[0003] For conventional SAR systems the coverage in across trackdirection and geometric resolution in along track direction arecontradicting system parameters.

[0004] In a conventional monostatic SAR system, the same real apertureof length L and height H is used for transmit and receive. In order tosample the radar echoes of the wanted target area unambiguously, it isshown in [1] that a minimum antenna aperture A is required.$\begin{matrix}{A = {{L \cdot H} > \frac{4 \cdot v_{s} \cdot \lambda \cdot R_{s} \cdot {\tan (\phi)}}{c}}} & {{Equ}.\quad 1}\end{matrix}$

[0005] In Equ. 1 v_(s) is the speed of the SAR platform, λ is thewavelength at center frequency, R_(s) is the slant range to the target,φ is the incidence angle and c is the speed of light. Even though Equ. 1is based on a number of approximations, it clearly shows the principlelimitations of a conventional SAR system. The two top level systemparameters swath width W_(SW) and the azimuth resolution δ_(az) arecontradicting and can not be improved at the same time: In order toilluminate a wider swath width, the antenna height H must be decreased.A better azimuth resolution in stripmap mode requires a shorter length Lof the antenna (δ_(az)=L/2).

[0006] For the case of an airborne SAR, this constraint is not soimportant because the platform speed v_(s) and the slant range R_(s) areorders of magnitude smaller, than in the spaceborne case. The minimumantenna size is a very important consideration in the spaceborne case.Conventional SAR systems use special modes of operation to overcomethese constrains. They are called the Spotlight and the ScanSAR mode[2].

[0007] The Spotlight mode allows to improve the azimuth resolution bypointing the antenna beam to the spot for a longer aperture. Thedisadvantage is that by doing so, only single high resolution spots canbe imaged, but no continuous coverage is possible.

[0008] The ScanSAR mode uses a highly agile antenna beam in order toswitch rapidly between a number of N subswath. This results in aimproved swath width but at the cost of a N+1 times reduced azimuthresolution.

[0009] DE 34 30 749 A1 describes a method of swath widening and datareduction in a SAR system. The method utilises the fact that the Dopplerhistory for targets from different distance ranges has slightdifferences. The echos, of targets from different distance ranges arereceived in one single receive channel and transmitted to the ground asjust one echo. There, the echos of different distance ranges can beseparated due to their individual Doppler histories.

[0010] The system described in [3] has a special mode for improved alongtrack resolution. During receive the aperture is divided in azimuth intotwo sub-apertures and the signal of each sub-aperture is separatelyrecorded and transmitted to the ground for SAR processing. The samedivision in azimuth can be used for the detection of moving targets.

[0011] The principle of moving target detection is described in detailin [4]. It requires multiple receive channels and multiple receiveantennas or sub-apertures separated in along track direction. Specialsignal processing algorithms allow then to detect moving targets withinthe SAR image.

[0012] A further technique using two receive apertures and receivechannels is the SAR interferometry [5]. There the two receive apertureshave to be separated in elevation or cross track direction. Theseparation required for interferometry is in the order of several tensor even hundreds of meters. Here again the two signals have to beseparately recorded and are combined only after the SAR imageprocessing.

[0013] The object of the present invention is to overcome the describedconstrains of a conventional SAR system. The new SAR system should allowto combine a high azimuth resolution with an improved swath width and acontinuous lossless coverage in stripmap mode.

[0014] According to the invention the SAR system is a bistatic radar,where the receive antenna is built up from multiple receivesub-apertures in azimuth as well as in elevation direction. A coherentdata processing to reduce the data volume is performed on board with thesignals from the sub-apertures.

[0015] The SAR system for spaceborne application is capable to combine avery high geometric resolution with a very large coverage area. Such aSAR system is e.g. well suited for large area surveillance and highresolution mapping applications. In particular, the SAR system accordingto the invention allows to combine a very high azimuth resolution withan improved swath width.

[0016] A higher coverage in across track as well as a higher geometricresolution in along track direction require both an increased averagetransmit power in a conventional SAR system. The SAR system according tothe invention allows to reduce the required average transmit power bythe use of a receive antenna with higher antenna gain and the optimizeddesign of the separated transmit and receive antennas.

[0017] The invention is described in more detail with reference to theaccompanying drawings, of which

[0018]FIG. 1 illustrates the principle of combining multiple aperturesin receive;

[0019]FIG. 2 shows multiple receive apertures in elevation,

[0020]FIG. 3 shows the effective phase center location when usingmultiple receive apertures in azimuth,

[0021]FIG. 4 shows multiple receive apertures in azimuth and elevation,

[0022]FIG. 5 is a bloc diagram showing the combination of signals fromdifferent receive apertures in elevation,

[0023]FIG. 6 illustrates the definition of angles and radii in a roundearth geometry,

[0024]FIG. 7 is a diagram depicting the array scan angle as a functionof the echo time,

[0025]FIG. 8 shows a possible embodiment of an receive aperture,

[0026]FIG. 9 shows a possible embodiment of a transmit aperture withtransmissions via a stacked horn array.

[0027] 2. Instrument Architecture

[0028] The SAR instrument architecture according to the inventioncombines a separate transmit aperture with multiple receive apertures inelevation as well as in azimuth. In the following first the principle ofmultiple sub-apertures in elevation is explained and then the principleof multiple sub-apertures in azimuth. Then both principle are combined.

[0029] The SAR instrument architecture of this invention has twoseparated apertures for transmit and receive. The two apertures of thebistatic radar can be either mounted on the same or on two differentsatellites flying in a constellation. This allows to optimize theelectrical design of each antenna and the RF electronic for transmittingor for receiving. The total aperture size can also be traded and variedbetween transmit and receive.

[0030] The size of the transmit aperture determines the target areailluminated with one radar pulse. The transmit aperture size inelevation is inversely proportional to the final image swath width. Inorder to produce a larger imaged swath width, the size of the transmitaperture in elevation h_(tx) must be smaller than in a conventional SARsystem. The azimuth dimension is proportional to the maximum receivableazimuth resolution.

[0031] To compensate for the smaller transmit gain, the total receiveaperture size in elevation is larger than in a conventional system. Itis divided into a number of sub-apertures. Each sub-aperture has tocover the area illuminated by the transmit aperture and therefore itselevation size h_(rx) has to be smaller or equal to the elevation sizeof the transmit aperture. The second requirement limiting the elevationsize of the sub-aperture is that when combining the signals of thesub-apertures, the generated quantisation lobes of the resulting antennapattern must be below a certain level. The azimuth size of each receivesub-aperture is the same as in the transmit aperture. To obtain the sameradiometric conditions as in a conventional system, the product of thetransmit and the total receive aperture size should be the same as thesquared conventional aperture size if all other instrument parametersremain the same. (It is expected that due to an optimized design of theseparate transmit and the receive path lower losses as well as a lowernoise figure can be achieved. This will in addition improve theefficiency of the SAR instrument and partially compensate fort the lowergain.)

[0032] The signal from each sub-aperture is received in a separatechannel. Each channel provides a separate input for the subsequentdigital signal processing.

[0033] 2.1. Multiple Receive Sub-Apertures in Elevation

[0034] The wide imaging swath and the high resolution in azimuthdirection require the use of a small transmit aperture compared to aconventional SAR system design as explained in the last paragraph.

[0035] This reduction of transmit aperture size causes a reducedreceived signal power (determined by the radar equation) compared to aconventional SAR system and so the radiometric resolution in the imageis decreased. To improve the radiometric performance the transmit powerand/or the effective receive aperture size can be increased. In thepresented concept the second is realized by building a larger receiveaperture in elevation out of multiple receive sub-apertures.

[0036] The echo received independently from each sub-aperture is phaseshifted by a time and frequency dependent phase function and thencoherently combined with the signals from the other receive apertures inthe preprocessing. If this preprocessing is realized on board of thesatellite, the resulting receive data rate is the same as for a radarwith one receive channel.

[0037] Effectively the processing of the signals of the multiple receiveapertures can be regarded as a multiple beam forming process were theresulting beams are steered between −3 dB points in elevation of thetransmit aperture as displayed in FIG. 1.

[0038] The steering of the beams during processing can createquantisation lobes in the resulting antenna receive pattern. This has tobe taken into account when the size in elevation of the receivesub-apertures h_(rs) is selected. In general the requirement on thereceive quantisation lobe level can be less stringent than in the caseof an active phased array antenna because the quantisation lobes onlyappear in the receive pattern and the independent transmit pattern canprovide a large portion of the alias signal suppression. In order tokeep the things simple for the explanation of the principles, onlyunweighted antenna apertures are investigated in the following. Theintroduction of an antenna taper gives additional freedom foroptimization.

[0039] The parameters to describe the multiple receive apertures inelevation configuration shown in FIG. 2 are listed in Tab. 1.

[0040] As previously described the elevation dimension of the receivesub-aperture h_(rs) relative to the elevation dimension of the transmitaperture h_(tx) is constrained by the highest specified grating lobelevel.

[0041] From the radar equation it is known that the signal to noiseratio for one of the observed target points is proportional to theproduct of transmit and receive gain which is again proportional to theproduct of transmit and receive aperture area. To be able to compare themultiple receive aperture configuration with the standard monostaticsingle transmit/receive aperture concept the effective antenna aperturecan be calculated.

SNR˜G ² ˜A ² =A _(tx) ·A _(rx) =lh _(tx) ·lh _(rx) =l ² ·h _(tx) ²

·α·N

  Equ. 2 $\begin{matrix}{{l \cdot h_{tx}} = \frac{A}{\sqrt{a \cdot N}}} & {{Equ}.\quad 3}\end{matrix}$

[0042] The total antenna aperture (transmit+receive) A_(bi) of abistatic configuration which is necessary to maintain the same SNR for apoint target when the other instrument parameters are unchanged iscalculated as follows. The parameter A=1·h is the corresponding antennasurface in a monostatic configuration which provides the same SNR (but asmaller swath width). $\begin{matrix}{{{\begin{matrix}{A_{bi} = {{l \cdot h_{tx}} + {l \cdot h_{rx}}}} \\{= {{l \cdot h_{tx}} + {l \cdot h_{rs} \cdot N}}} \\{= {{l \cdot h_{tx}} + {l \cdot h_{tx} \cdot a \cdot N}}} \\{= {l \cdot h_{tx} \cdot ( {1 + {a \cdot N}} )}}\end{matrix}\quad {where}\text{:}\quad N} > 1};{N \in N}} & {{Equ}.\quad 4}\end{matrix}$

[0043] When inserting Equ. 2 the following is obtained. $\begin{matrix}{{A_{bi} = {{\frac{A}{\sqrt{a \cdot N}} \cdot ( {1 + {a \cdot N}} )} = {A \cdot ( {\frac{1}{\sqrt{a \cdot N}} + \sqrt{a \cdot N}} )}}}{A_{tx} = {A \cdot \frac{1}{\sqrt{a \cdot N}}}}{A_{rx} = {A \cdot \sqrt{a \cdot N}}}} & {{Equ}.\quad 5}\end{matrix}$

[0044] In this simplified model it can be assumed that the imaged swathwidth in ground range direction is proportional to the antenna beamwidth in elevation. This means that the swath width, which can beimaged, is inverse proportional to the transmit aperture dimension inelevation h_(tx).

[0045] From Equ. 5 it can be seen how the transmit aperture heighth_(tx), the receive aperture height h_(rx) and from this also the imagedswath width are related as a function of N when taking the monostaticantenna height h and the corresponding swath width as the reference.$\begin{matrix}{{h_{tx} = {h \cdot \frac{1}{\sqrt{a \cdot N}}}}{h_{rx} = {h \cdot \sqrt{a \cdot N}}}{\frac{1}{h_{tx}} = {\frac{1}{h} \cdot \sqrt{a \cdot N}}}} & {{Equ}.\quad 6}\end{matrix}$

[0046] The swath width to be imaged is increased by a factor of$\frac{h}{h_{tx}} = \sqrt{a \cdot N}$

[0047] when the transmit aperture dimension in elevation h_(tx) isdecreased, which is proportional to the increase of the receive apertureheight h_(rx).

[0048] This leads to the conclusion that different to the monostatic SARsystems in the proposed bistatic SAR system concept, the imaged swathwidth is increased proportional to the receive aperture height h_(rx)without decreasing the radiometrc and geometric resolution.

[0049] 2.2. Limitations of the Swath Width Due to Ambiguities in Range

[0050] One factor which limits the maximum achievable swath width inrange are the range ambiguities. Ambiguities in range occur when theantenna receives at the same time echos which are generated fromsubsequent pulses and which therefore cannot be distinguished. This canhappen in the spaceborne case where always a number of subsequent pulsesare in the ‘air’ at the same time. The distance between these pulses canbe increased by reducing the Pulse Repetition Frequency (PRF). But thereis a lower limit for the PRF because the azimuth spectrum has to besampled correctly. A good rule of thumb is that the next pulse is sentat the latest when the satellite has moved forward by ½ of the antennaazimuth length l. With this the minimum PRF is defined in Equ. 7.$\begin{matrix}{{PRF}_{\min} = \frac{2\quad v_{s}}{l}} & {{Equ}.\quad 7}\end{matrix}$

[0051] The worse case, that is the minimum achievable swath width W_(SW)for a certain PRF, can be found at the maximum incidence angle φ_(i) ascan be seen in Equ. 8. The incidence angle φ_(i) is defined as the anglebetween the local normal vector on the earth surface and the directionfrom were the wave is approaching. Further it is assumed that only amaximum of 80% of the time intervals between the pulses can be used forreceiving the echoes. The remaining time is reserved for the transmitpulse and some guard time. The speed of light is denoted as c₀.$\begin{matrix}\begin{matrix}{w_{sw} = {0.8\quad \frac{c_{0}}{{2 \cdot {PRF} \cdot \sin}\quad \phi_{i}}}} \\{= {0.8\quad \frac{l \cdot c_{0}}{{4 \cdot v_{s} \cdot \sin}\quad \phi_{i}}}}\end{matrix} & {{Equ}.\quad 8}\end{matrix}$

[0052] Equ. 8 is an approximation as it assumes a constant incidenceangle over the whole swath width.

[0053] 2.3. Multiple Receive Sub-Apertures in Azimuth

[0054] This conflict between the high azimuth resolution and the largeswath width can be solved with a configuration, where multiple (e.g. M)receive apertures are placed in azimuth direction. This arrangementenables correct sampling of the azimuth spectrum with a PRF fitting thetotal antenna azimuth dimension, which is M times smaller than the PRFnecessary for the sub-aperture size I. This is possible because withevery pulse the echo is sampled at M different positions. As theeffective phase center is located in the middle between the transmit andthe receive aperture the maximum azimuth sample spacing of I/2 isfulfilled. This displaced phase center system operation [2] isdemonstrated in FIG. 3.

[0055] In order to fulfill the radiometric requirements each receiveaperture must have the aperture size as derived in paragraph 2.1.

[0056] The echo received in each sub-aperture must be stored separately.

[0057] The concept of multiple receive sub-apertures in elevation andmultiple sub-apertures in azimuth can be combined. Such a configurationis shown in FIG. 4.

[0058] The configuration with multiple receive apertures in elevationand in azimuth can be characterized with the set of five parametersshown in Tab. 2.

[0059] The receive aperture has the dimension L=M·l in azimuth andh_(rx)=N·h_(rs) in elevation.

[0060] 2.4. Selection of the Region of Interest

[0061] The steering to the region of interest can be realized by a rollof the satellite. It is assumed that the swath width in strip map mode,which is larger than with a conventional SAR system, is sufficient andso no extra ScanSAR mode is needed for wide swath application. Due tothe mechanical pointing the selection of a new swath at a differentrange position needs more time than with a SAR system, which is equippedwith an active phased array antenna.

[0062] 3. On-Board Signal Processing

[0063] The presented concept requires to have multiple receivesub-apertures which provide one receive signal each. This would increasethe amount of data, which has to be stored on board and then transmittedto the ground by (N·M) the number of receive sub-apertures. This isespecially critical for a high resolution wide swath radar system, whichin any case requires to handle a large data volume. In the processing ofthe SAR data to an image the different receive signals have to becombined. It is suggested that at least this part of the processing isdone on board in order to reduce the data volume.

[0064] The concept foresees multiple receive sub-apertures in elevationas well as in azimuth. The multiple receive sub-apertures in azimuth areused to reduce the required PRF. Due to this the multiple of receiveapertures in azimuth do not increase the effective data rate and thereis no data reduction possible.

[0065] This is different from the multiple receive sub-apertures inelevation located in one column. It is possible to combine the signalsfrom different receive sub-apertures within a column into one signalthat contains all necessary information. In principle it is done by atime variant phase shift of each signal followed by the summation of theecho signals. This phase shift can be realized digitally e.g. by a timevariant phase multiplication. The time variant phase shift is performedin a way that the radar echo signal is maximized in the summed signal asthe radar signal runs over the earth's surface.

[0066] After this summation of the N signals from the N sub-apertures inthe column only the M signals from the M columns have to be stored onboard and then transmitted to the ground. This corresponds to a datareduction by a factor N.

[0067] For radar systems, which use a chirped signal instead of a shortpulse, the correct time variant phase shift varies with frequencybecause of the longer duration of the transmit pulse and the linear timeconnection between time and frequency in a linear chirp. This means thatadditionally to the time variant phase shift a time invariant butfrequency dependent phase shift is required. Again the objective of thisadaptive time-frequency variant processing is to maximize radar echosignal power in the resulting signal. The frequency variant phase shiftcan be realized for example with a specially designed allpass filter orby a phase multiplication after transformation in the frequency domain.

[0068] The block diagram in FIG. 5 shows the signal processingoperations and the signal combination from the different receivesub-apertures in elevation. The processing of the signals is bestimplemented digitally following an analog to digital conversion of thesignal form each sub-aperture so that no beam-forming network inelevation is required.

[0069] The described signal processing could be implemented with analgorithm containing the following steps:

[0070] 1. Implementation of a time variant phase shift by multiplicationof an adapted time variant phase value to the signal of eachsub-aperture according to the time variant echo direction and thesub-aperture's position in the receive aperture.

[0071] 2. Correction of this time variant phase shift for the differenttimes in the transmit pulse by introduction of a adapted frequencydependent phase shift to each echo signal. This frequency variant phaseshift is adapted to the radar geometry and the position in the receivesub-aperture. One way of realization is to design the transfer functionof an allpass filter accordingly.

[0072] 3. Coherent summation of the signals from the sub-apertures inone column into one single signal with maximized processing gain for therelevant echo signal.

[0073] 4. Optionally, the resulting signal can be compressed using a BAQlike algorithm (BAQ Method of Block Adaptive Quantization [2]).

[0074] 5. With this signal a conventional SAR image processing has to beperformed in order to form SAR images from the raw data.

[0075] 3.1. Derivation of the Time Variant Phase Shift

[0076] A locally round earth with a local earth radius R_(E), as shownin FIG. 6, is assumed. The other parameters are the orbit radiusR_(orbit), the incidence angle at bore sight of the, antenna φ_(i), the−3 dB width of the transmit antenna θ_(tx), the wavelength at centerfrequency λ and the distance of the phase centers of the sub-aperturesin elevation corresponding to their height h.

[0077] First the situation for a short pulse SAR system is examined. Thereceive signal timing is characterized by the swath center echo timet_(echo) and the data window length t_(data).

[0078] The off nadir angle β for a given incidence angle φ _(i) is givenby the following equation.$\beta = {\arcsin ( \frac{R_{E}\quad \sin \quad \phi_{i}}{R_{Orbit}} )}$

[0079] The bore sight of the antenna points in direction of the offnadir angle β. The limits of the imaged swath in terms of the off nadirangle are defined by the 3 dB width θ_(tx) of the transmit antenna. Theoff nadir angle for the near range swath edge is β_(n)=β−θ_(tx)/2 andβ_(f)=β+θ_(tx)/2 for the far range swath edge.

R _(s) =R _(Orbit)·cos(β)−{square root}{square root over (R _(Orbit)²·(cos²(β)−1)+R _(E) ²)}  Equ. 10

[0080] The slant range R_(s) can be converted into an echo time bydividing with the half of the speed of light t=2R_(s)/c₀. Combining thisthe near range time t_(n) and the far range time t_(f) are defined bythe following equations. $\begin{matrix}{{t_{n} = {2 \cdot \frac{\begin{matrix}{{R_{Orbit} \cdot {\cos ( {\beta - \frac{\theta_{tx}}{2}} )}} -} \\{\quad \sqrt{{R_{Orbit}^{2} \cdot ( {{\cos^{2}( {\beta - \frac{\theta_{tx}}{2}} )} - 1} )} + R_{E}^{2}}}\end{matrix}}{c_{0}}}}{t_{f} = {2 \cdot \frac{\begin{matrix}{{R_{Orbit} \cdot {\cos ( {\beta + \frac{\theta_{tx}}{2}} )}} -} \\{\quad \sqrt{{R_{Orbit}^{2} \cdot ( {{\cos^{2}( {\beta + \frac{\theta_{tx}}{2}} )} - 1} )} + R_{E}^{2}}}\end{matrix}}{c_{0}}}}} & {{Equ}.\quad 11}\end{matrix}$

[0081] The echo window time, which has to be sampled is simply given bythe difference t_(echo)=t_(f)−t_(n). The swath center echo time is givenby t_(center)=(t_(f)+t_(n))/2.

[0082] In the next step the angle θ in which the resulting receivepattern has to be steered away from the antenna bore sight must bedetermined as a function of echo time. For this Equ. 12 has to beinverted. $\begin{matrix}{{t(\theta)} = {2 \cdot \frac{{R_{Orbit} \cdot {\cos ( {\beta + \theta} )}} - \sqrt{{R_{Orbit}^{2} \cdot ( {{\cos^{2}( {\beta + \theta} )} - 1} )} + R_{E}^{2}}}{c_{0}}}} & {{Equ}.\quad 12}\end{matrix}$

[0083] The inversion is given in Equ. 13. $\begin{matrix}{{\theta (t)} = {{a\quad {\cos ( \frac{{4 \cdot R_{Orbit}^{2}} - {4 \cdot R_{E}^{2}} + {t^{2}c_{0}^{2}}}{4 \cdot R_{Orbit} \cdot t \cdot c_{0}} )}} - \beta}} & {{Equ}.\quad 13}\end{matrix}$

[0084] The angle θ as a function of the echo time is displayed in FIG. 7using a set of realistic parameters: φ_(i)=45°, θ_(tx)=3.8° (correspondsto 0.1 m t_(X) antenna height), R_(Orbit)=7038 km, R_(E)=6378 km. Herethe relation between the echo time and the array scan angle resembles alinear function. For wider swath and larger θ_(tx) the non linearity dueto the earth curvature becomes more visible.

[0085] In order to find a linear approximation of the time variant scanangle θ(t) Equ. 13 is differentiated with respect to the time δθ(t)/δtand the expression is evaluated at the swath center time t_(c).$\begin{matrix}{{\frac{\partial\quad}{\partial t}{\theta ( t_{c} )}} = \frac{{- \frac{1}{2}}{c_{0}( {{R_{Orbit}^{2}{\cos^{2}(\beta)}} - {R_{Orbit}{\cos (\beta)}\sqrt{{R_{Orbit}^{2}{\cos^{2}(\beta)}} - R_{Orbit}^{2} + R_{E}^{2}}} - R_{Orbit}^{2} + R_{E}^{2}} )}}{R_{Orbit}^{2}{\sin (\beta)}( {{2R_{Orbit}^{2}{\cos^{2}(\beta)}} - {2R_{Orbit}{\cos (\beta)}\sqrt{{R_{Orbit}^{2}{\cos^{2}(\beta)}} - R_{Orbit}^{2} + R_{E}^{2}}} - R_{Orbit}^{2} + R_{E}^{2}} )}} & {{Equ}.\quad 14} \\{t_{c} = {2 \cdot \frac{{R_{Orbit} \cdot {\cos (\beta)}} - \sqrt{{R_{Orbit}^{2} \cdot ( {{\cos^{2}(\beta)} - 1} )} + R_{E}^{2}}}{c_{0}}}} & {{Equ}.\quad 15}\end{matrix}$

[0086] In FIG. 7 the function θ(t) from Equ. 13 and the linearapproximation are shown.

[0087] 3.1.1. Scan Angle of the Different Receive Sub-Apertures

[0088] In the next step the scan angle of the receive aperture must beconverted into a phase shift in the individual signals coming from thesub-arrays. This analysis is done the same way as for an electronicallyscanned array. Instead of realizing the phase shift in analog in the RFband it is realized digitally in the complex equivalent basis band.

[0089] The phase center position is assumed to be located in the centerof the sub-arrays and d_(n) is the directed distance of the n^(th)sub-arrays phase center from the center of the receive aperture, whichis positive if located above the center of the aperture. Together withthe radar wavelength λ the actual phase shift γ_(n) to be implementedfor the signal of each sub-aperture as a function of the steer angle θcan be calculated with Equ. 16.

γ_(n)(t)=d _(n)·sin(θ(t))·2π/λ  Equ. 16

[0090] 3.2. Derivation of the Time-Frequency Variant Phase Shift for aChirped SAR System

[0091] The time variant phase shift, which was given in chapter 3.1 hasto be extended by a frequency dependent phase shift when a frequencymodulated transmit signal is used instead of short pulse to obtain therequired range resolution.

[0092] The SAR system operating with a linear chirp is describedadditionally to the already given parameters with its pulse lengthτ_(p), SAR signal bandwidth B, the chirp rate K=B/τ_(p) and the samplingfrequency f_(s) of the A/D converter. In a chirped system the echocoming from the imaged swath is extended by the pulse length. Therefore,in this case, the echo window is given by t_(echo)=t_(f)−t_(n)+τ_(p).

[0093] For the short pulse system the resulting receive beam pattern canalways be pointed to the direction from which the echo originates. Inthe case of a long pulse this is only possible for one position in thepulse. The rest of the pulse does not receive the full receive gain ofthe antenna. This can be compensated when additionally a frequencydependent beam steering is added. After the phase multiplication in thetime domain, which follows the center of the pulse, a second phasemultiplication in the spectral domain which implements to proper beamsteering for every section within the pulse. This is possible because ofthe direct connection between the time and the frequency present in thelinear chirp signal.

[0094] The following steps are necessary to combine the echo signalsfrom the different sub-apertures in the chirped signal case:

[0095] 1. Multiplication of the time signal with a time variant phasefunction γ_(n)(t) for every sub-aperture to realize the aperture steerangle θ_(ch)(t). The difference to the steer angle of the short pulsesystem shown in Equ. 13 is that the steer angle is calculated for themiddle of the pulse which corresponds to the center frequency of thechip. The steer angle of the chirped system θ_(ch)(t) shown in Equ. istherefore given by a τ_(p)/2 delayed version of θ(t). $\begin{matrix}{{\theta_{ch}(t)} = {{\theta ( {t - \frac{\tau_{p}}{2}} )} = {{a\quad {\cos ( \frac{{4 \cdot R_{Orbit}^{2}} - {4 \cdot R_{E}^{2}} + {( {t - \frac{\tau_{p}}{2}} )^{2}c_{0}^{2}}}{4 \cdot R_{Orbit} \cdot ( {t - \frac{\tau_{p}}{2}} ) \cdot c_{0}} )}} - \beta}}} & {{Equ}.\quad 17}\end{matrix}$

[0096] 2. Realizing a scan angle θ_(f)(f) for the frequencies −B/2≦f≦B/2to compensate the spread of the signal over time of the chirp.$\begin{matrix}{{\theta_{f}(f)} = {{\frac{\partial\quad}{\partial t}{{\theta ( t_{c} )} \cdot \frac{\tau_{p}}{B} \cdot f}} = {\frac{\partial\quad}{\partial t}{{\theta ( t_{c} )} \cdot \frac{f}{\kappa}}}}} & {{Equ}.\quad 18}\end{matrix}$

[0097] 3. The steering angles θ_(ch)(t) and θ_(f)(f) are converted intophase values for multiplication of signal or the spectrum of the signalof each sub-aperture with the help of Equ. 16. The modification ofspectrum with a linear phase can be realized with a time delay of thesignal adapted for each sub-aperture. The parts of the time delaycorresponding to full sampling periods can be realized by storing thedata for a number of clock cycles. The parts of the time delaycorresponding to fractions of a sampling period can be realized by aninterpolation of the data. One way to realize this interpolationdigitally is by using an interpolation filter. The sub-sampling perioddelay can be also realized by a shift of the clock signal for the analogto digital converter.

[0098] 4. Possible Technological Implementation

[0099] 4.1. Technologies for Receive Aperture

[0100] The total receive aperture can become a relatively largestructure, which must be storable for the launch and then deployment inspace. For this purpose it is important that the structure is rigid,light weight and possibly thin. The electrical requirements are a highbandwidth and a low electric loss in front of the low noise amplifier tokeep the total system losses and the system noise figure low and thecost for production shall be low.

[0101] A microstrip patch radiator has a very good potential to fulfillthese requirements. Behind a certain number of patches a low noiseamplifier amplifies the signal transmission to the central RFelectronic. The electric power consumption of a receive path isrelatively low so that a temperature stabilization of the LNA can beconsidered to encounter phase variation. The number of LNAs persub-aperture is determined by the tolerable losses in front of the LNAto meet the required noise figure and by the required output power ofone sub-aperture.

[0102] The radiator could be mounted on a honeycomb carbon-fibersandwich, which provides the necessary mechanical support (see FIG. 8).In a first estimate the described structure in X-band would weight about8 kg/m² and is less then 30 mm thick.

[0103] 4.2. Technologies for Transmit Aperture

[0104] Compared to the receive aperture, the transmit aperture isrelatively small. The main emphasis is to radiate the generated RFenergy with as low losses as possible. For the generation of the RFenergy Traveling Wave Tubes (TWT) could be used as well as Medium PowerModules (MPM).

[0105] Depending on the system layout, the sub-apertures are much longerin azimuth direction then in elevation. This requires a specializedreflector design with possibly multiple feeds. An alternative would beto radiate directly from multiple horns. Together with a larger numberof MPMs they could be stacked to built up the transmit aperture (seeFIG. 9). A large enough number of these units could provide gracefuldegradation as a redundancy concept.

5. REFERENCES

[0106] [1] J. C. Curlander and R. N. McDonough, Synthetic Aperture RadarSystems and Signal Processing, New York: Wiley, 1991 p. 21 ff.

[0107] [2] A. Currie, M. A. Brown, Wide-swath SAR, IEE Proceedings-F,Vol. 139, No. 2, April 1992

[0108] [3] R. Kwok, W.T.K. Johnson, Block Adaptive Quantization ofMagellan SAR Data, 1989, IEEE Trans. Geoscience & Remote Sensing, Vol.27, No. 4, pp. 375-383

[0109] [4] P. Meisl, A. Thompson, A. Luscombe, RADARSAT-2 Mission:Overview and Develop-ment Status, Proceedings of EUSAR 2000[5] J. H. G.Ender, Detection and Estimation of Moving Target Signals byMulti-Channel SAR, AEÜ Int. J. Electron Commun. 50(1996) No. 2, 150-156

[0110] [5] Fuk K. Li, R. M. Goldstein, Studies of MultibaselineSpaceborne Interferometric Synthetic Aperture Radars, IEEE Transactionson Geoscience and Remote Sensing, Vol. 28, No. 1, January 1990

[0111] 6. Tables TABLE 1 Parameters describing the multiple receiveapertures in elevation configuration Parameter Description Unit Azimuthdimension of one sub-aperture [m] I Elevation dimension of the transmitaperture [m] h_(tx) Elevation dimension of one receive sub-aperture [m]h_(rs) Number of sub-apertures attached in elevation in the N receiveaperture Elevation dimension of the total receive aperture [m] h_(rx) =N h_(rs)

[0112] TABLE 2 Parameters characterizing a recieve aperture withmultiple sub-apertures in elevation and in azimuth Parameter DescriptionUnit Azimuth dimension of one sub-aperture [m] I Elevation dimension ofthe transmit aperture [m] h_(tx) Elevation dimension of one receivesub-aperture [m] h_(rs) Number of sub-apertures attached in elevation inthe N receive aperture Elevation dimension of the total receive aperture[m] h_(rx) = N · h_(rs) Number of sub-apertures attached in azimuth inthe M receive aperture Azimuth dimension of the total receive aperture[m] M · I

1. Side-looking SAR system, comprising a transmit aperture, a receiveaperture of different size, separated from said transmit aperture anddivided into a number of receive sub-apertures arranged along elevationand azimuth direction, means for coherently processing the signals ofeach receive sub-aperture comprising means for phase shifting the signalfrom each receive sub-aperture by a time and/or frequency variant phasevalue, means for the summation of the resulting signals from receivesub-apertures arrayed along the elevation direction, whereby the timeand/or frequency variant phase value is generated in such a way that theradar echo signal is maximized in the summed signal as the radartransmit signal runs over the earth's surface.
 2. Side-looking SARsystem according to claim 1, characterized in that transmit aperture andreceive aperture are mounted on the same or on two different satellitesflying in constellation.
 3. Side-looking SAR system according to any oneof the preceding claims characterized in that in case of using a chirpedSAR radar transmit signal the means for phase shifting the signal fromeach receive sub-aperture comprise means for multiplication of a timevariant phase value to the signal of each receive sub-aperture; meansfor imposing a frequency variant phase value on the resulting signal inorder to take into account the variant dependence between time,frequency and direction in case of a SAR radar using a chirped transmitsignal.
 4. Side-looking SAR-System according to claim 3, characterizedin that said means for imposing a frequency variant phase value comprisemeans for imposing time delays of rational multiples of the samplingperiod on the resulting signal from each receive sub-aperture. 5.Side-looking SAR-System according to claim 4, characterized in that saidmeans for imposing time delays comprise means for controlled shifting ofthe clock signal of the analogue to digital converter, means of delayingsignal samples by a number of full sampling periods.
 6. Side-looking SARsystem according to any one of the preceding claims further comprisingmeans for data compression of the summed signals of differentsub-apertures by using a BAQ like algorithm.
 7. Side-looking SAR systemaccording to any one of the preceding claims characterized in that thetransmit aperture is not divided into any sub-apertures.